1. Field of the Invention
The present invention relates to an improved process for preparing diphosphines such as bis(diarylphosphinomethyl)arenes.
2. Description of the Background
Phosphines have many applications in industry. They have industrial importance as antioxidants, metal extractants, flame retardant impregnants, stabilizers for olefins, starting materials for Wittig reagents and, in particular, as ligands for transition metal catalysts.
A review of the important homogeneous catalysts containing phosphine ligands may be found, for example, in B. Cornils, W. A. Herrmann, Applied Homogeneous Catalysis with Organometallic Compounds Vol. 1 & 2, VCH, Weinheim, 1996. Within the group of phosphines, chelating phosphines, in particular diphosphines, are of particular importance as ligands for metal complexes. This is due to the fact that diphosphines form more stable complexes with the appropriate central metal atom and therefore have a more lasting influence on its catalytic properties. Among the known chelating diphosphines, bis(diarylphosphinoalkyl)-1,1′-binaphthyls as constituents of carbonylation catalysts and telomerization catalysts are of industrial interest. EP 0 653 432, EP 0 673 944 and JP 7939059 describe the preparation of bis(diarylphosphinoalkyl)-1,1′-binaphthyls by reaction of 2,2′-dimethylbinaphthyls with a brominating agent to form 2,2′-bis(bromomethyl)-1,1′-binaphthyl, subsequent reaction with an alkyl diphenylphosphinite and subsequent reduction of the bisphosphine oxide formed by means of an organodichlorosilane. This process has a number of fundamental disadvantages: to introduce the phosphorus groups into the target molecules, the latter firstly have to be brominated, and the bromine atoms are then replaced by phosphorus groups. This requires the use of expensive and toxic bromination reagents and results in the formation of superstoichiometric amounts of bromide waste. The bromination of the starting compounds generally leads to product mixtures whose handling is problematical since the arylalkyl bromides formed are lacrymatory substances which are hazardous to health. In addition, the reaction with alkyl diphenylphosphinites leads to the phosphine oxide which has to be reduced in a further step. Starting out from 2,2′-dimethylbinaphthyls, 4 steps are thus necessary to obtain the desired ligands.
Similar syntheses or partial syntheses having the same and sometimes further disadvantages are also described in the literature (M. E. Jung et al., Tetrahedron Lett. 1988, 29, 6199; H. J. Bestmann et al., Chem. Ber. 1974, 2926; T. Hayashi et al., J. Am. Chem. Soc. 1988, 110, 8153).
In EP 0 704 449, unsymmetrically substituted bis(diarylphosphinoalkyl)-1,1′-binaphthyls are prepared from 2,2′-bis(bromomethyl)-1,1′-binaphthyl via corresponding phosphonium salts. This process has the abovementioned disadvantages and is not advantageous for the preparation of symmetrical bis(diarylphosphinoalkyl)-1,1′-binaphthyls since the phosphorus groups have to be introduced into the target molecule in two steps and large amounts of salt waste are formed.